Heart disease is one of the most serious health concerns in the western world. It is estimated that 61 million Americans (nearly 1 in 5 men and women) have one or more types of cardiovascular disease (National Health and Nutrition Examination Survey III, 1988-94, Center of Disease Control and the American Heart Association). Widespread conditions include coronary heart disease (12.4 million), congenital cardiovascular defects (1 million), and congestive heart failure (4.7 million). A central challenge for research in regenerative medicine is to develop cell compositions that can help reconstitute cardiac function in these conditions.
Most of the research work done so far has been performed using stem cells of various kinds developed using rodent animal models.
Maltsev, Wobus et al. (Mechanisms Dev. 44:41, 1993) reported that embryonic stem (ES) cells from mice differentiated in vitro via embryo-like aggregates into spontaneously beating cardiomyocytes. Wobus et al. (Ann. N.Y. Acad. Sci. 27:460, 1995) reported that pluripotent mouse ES cells reproduce cardiomyocyte development from uncommitted embryonal cells to specialized cellular phenotypes of the myocardium. Embryoid bodies were plated, cultured, dissociated, and assayed by immunofluorescence and electrophysiological studies. The cells were reported to express cardiac-specific genes and all major heart-specific ion channels. Wobus et al. (J. Mol. Cell. Cardiol. 29:1525, 1997) reported that retinoic acid accelerates ES cell-derived cardiac differentiation and enhances development of ventricular cardiomyocytes. The investigation used cell clones transfected to express β-galactosidase under control of the MLC-2v promoter.
Kolossov et al. (J. Cell Biol. 143:2045, 1998) reported isolation of cardiac precursor cells from mouse ES cells using a vector containing green fluorescent protein under control of the cardiac α-actin promoter. Patch clamp and Ca++ imaging suggested expression of L-type calcium channels starting from day 7 of embryoid body development. Narita et al. (Development 122:3755, 1996) reported cardiomyocyte differentiation by GATA-4 deficient mouse ES cells. In chimeric mice, GATA-4 deficient cells were found in endocardium, myocardium and epicardium. The authors proposed that other GATA proteins might compensate for lack of GATA-4.
U.S. Pat. No. 6,015,671 (Field) and Klug et al. (J. Clin. Invest. 98:216, 1996) reported that genetically selected cardiomyocytes from differentiating mouse ES cells form stable intracardiac grafts. Cells were selected from differentiating murine ES cells using the α-cardiac myosin heavy chain (MHC) promoter driving aminoglycoside phosphotransferase or neor, and selecting using the antibiotic G418. Following transplantation into the hearts of adult dystrophic mice, labeled cardiomyocytes were reportedly found as long as 7 weeks after transplantation. International patent publication WO 00/78119 (Field et al.) proposes a method for increasing proliferative potential of a cardiomyocyte by increasing the level of cyclin D2 activity.
Doevendans et al. (J. Mol. Cell Cardiol. 32:839, 2000) proposed that differentiation of cardiomyocytes in floating embryoid bodies is comparable to fetal cardiomyocytes. Rodent stem cell derived cardiomyocytes were reported to differentiate into ventricular myocytes having sodium, calcium, and potassium currents.
Muller et al. (FASEB J. 14:2540, 2000) reported the isolation of ventricular-like cardiomyocytes from mouse ES cells transfected with green fluorescent protein under control of the ventricular-specific 2.1 kb myosin light chain-2v promoter and the CMV enhancer. Electrophysiological studies suggested the presence of ventricular phenotypes, but no pacemaker-like cardiomyocytes. Gryschenko et al. (Pflugers Arch. 439:798, 2000) investigated outward currents in mouse ES cell derived cardiomyocytes. The predominant repolarizing current in early-stage ES-derived cardiomyocytes was 4-aminopyridine sensitive transient outward current. The authors concluded that in early stage cardiomyocytes, this transient outward current plays an important role in controlling electrical activity.
International patent publication WO 92/13066 (Loyola University) reported the construction of rat myocyte cell lines from fetal material genetically altered with the oncogenes v-myc or v-ras. U.S. Pat. Nos. 6,099,832 and 6,110,459 (Mickle et al., Genzyme) report on the use of various combinations of adult cardiomyocytes, pediatric cardiomyocytes, fibroblasts, smooth muscle cells, endothelial cells, or skeletal myoblasts to improve cardiac function in a rat model. U.S. Pat. No. 5,919,449 (Diacrin) reports on the use of pig cardiomyocytes for treating cardiac insufficiency in a xenogeneic subject. The cells are obtained from an embryonic pig between ˜20-30 days gestation.
Makino et al. (J. Clin. Invest. 103:697, 1999) and K. Fukuda (Artificial Organs 25:1878, 2001) developed regenerative cardiomyocytes from mesenchymal stem cells for cardiovascular tissue engineering. A cardiomyogenic cell line was developed from bone marrow stroma, and cultured for more than 4 months. To induce cell differentiation, cells were treated with 5-azacytidine for 24 hours, which caused 30% of the cells to form myotube-like structures, acquire cardiomyocyte markers, and begin beating.
Most established cardiomyocyte lines have been obtained from animal tissue. There are no established cardiomyocyte cell lines that are approved for widespread use in human cardiac therapy.
Liechty et al. (Nature Med. 6:1282, 2000) reported that human mesenchymal stem cells engraft and demonstrate site-specific differentiation after in utero transplantation into sheep. Long-term engraftment was reportedly achieved for as long as 13 months after transplantation, which is after the expected development of immunocompetence. International patent publication WO 01/22978 proposes a method for improving cardiac function in a patient with heart failure, comprising transplanting autologous bone marrow stroma cells into the myocardium to grow new muscle fibers.
International patent publication WO 99/49015 (Zymogenetics) proposes the isolation of a nonadherent pluripotent cardiac-derived human stem cell. Heart cells are suspended, centrifuged on a density gradient, cultured, and tested for cardiac-specific markers. Upon proliferation and differentiation, the claimed cell line produces progeny cells that are fibroblasts, muscle cells, cardiomyocytes, keratinocytes, osteoblasts, or chondrocytes.
It is unclear whether any of the cell preparations exemplified in these publications can be produced in sufficient quantities for mass marketing as a therapeutic composition for regenerating cardiac function.
A more promising source of regenerative cells for treating cardiac disease is human pluripotent stem cells obtained from embryonic tissue.
Thomson et al. (Proc. Natl. Acad. Sci. USA 92:7844, 1995) were the first to successfully culture embryonic stem cells from primates, using rhesus monkeys and marmosets as a model. They subsequently derived human embryonic stem (hES) cell lines from human blastocysts (Science 282:114, 1998). Gearhart and coworkers derived human embryonic germ (hEG) cell lines from fetal gonadal tissue (Shamblott et al., Proc. Natl. Acad. Sci. USA 95:13726, 1998). International Patent Publication WO 00/70021 refers to differentiated human embryoid cells, and a method for producing them from hES cells. International Patent Publication WO 01/53465 outlines the preparation of embryoid body-derived cells from hEG cells.
Both embryonic stem cells and embryonic germ cells can proliferate in vitro without differentiating, they retain a normal karyotype, and they retain the capacity to differentiate to produce a variety of adult cell types. However, it is clear that the propagation and differentiation of human pluripotent stem cells is subject to very different rules than what has been developed for the culture of rodent stem cells.
Geron Corporation has developed novel tissue culture environments that allow for continuous proliferation of human pluripotent stem cells in an environment essentially free of feeder cells. See Australian patent AU 729377, and International Patent Publication WO 01/51616. Being able to culture stem cells in a feeder-free environment provides a system in which cellular compositions can be readily produced that are in compliance with the regulatory requirements for human therapy.
In order to realize the potential of pluripotent stem cells in the management of human health and disease, it is now necessary to develop new paradigms to drive these cells into populations of therapeutically important tissue types.